To increase heat resistance of acrylonitrile-butadiene-styrene (hereinafter referred to as “ABS”) resin, a method of substituting a portion or the entirety of styrene with α-methyl styrene and using emulsion polymerization has been generally used. A heat-resistant SAN resin is prepared based on α-methyl styrene. When such a heat-resistant SAN resin is prepared using emulsion polymerization, high glass transition temperature and high molecular weight may be obtained compared to bulk polymerization, whereby superior heat resistance and environmental stress cracking resistance (ESCR) are exhibited. In particular, to adjust glass transition temperature as an important factor determining heat resistance of a heat-resistant resin, the content of α-methyl styrene is increased or molecular weight is increased. When the content of α-methyl styrene is increased, reactivity of α-methyl styrene per se is low and thus a proportion of unreacted monomers increases, which causes heat resistance deterioration. In addition, since a separate device for removing unreacted monomers is required, product costs increase, productivity is decreased, polymerization time increases, and a conversion rate is decreased. In addition, when molecular weight is increased, fluidity of a final product is decreased, whereby there may be limitations in increasing molecular weight. Further, when molecular weight is decreased by increasing the content of molecular weight adjuster to improve fluidity, odor may be generated during processing of a mercaptan mainly used as a molecular weight adjuster.
To address these problems, KR 10-1993-0021665 A discloses a method of emulsion-polymerizing an α-alkyl styrene-acrylonitrile copolymer to prepare a heat-resistant SAN copolymer. However, when this method is used, a long polymerization time of nine hours or more is required, whereby copolymer productivity is decreased.
In addition, KR 10-1996-0031486 A discloses a method of adding an electrolyte to prepare a heat-resistant SAN copolymer having a high latex solid content and superior stability using emulsion polymerization. However, this method also requires a long polymerization time of six hours or more, whereby copolymer productivity is decreased.